1. Field of the Invention
The present invention relates to apparatus for electrolyzing an electrolytically regeneratable substance included in a liquid, and in particular to apparatus for elecctrolyzing an electrolytically regeneratable inonic oxidizing agent included in an aqueous acid stream.
2. Description of the Prior Art
Commonly assigned U.S. Pat. No. 3,793,171 entitled "Process for Removing Pollutants from Gas Streams," which issued on Feb. 19, 1974, is directed to a process for removing oxidizable impurities from a gas stream by contacting the gas stream in a mass transfer relationship with an aqueous acid stream containing an electrolytically regeneratable ionic oxidizing agent, so that the oxidizing agent reacts with the impurities to destroy the latter. The oxidizing capacity of the ionic oxidizing agent is reduced by the preceding reaction because the valence state of the oxidizing agent decreases. Therefore, the ionic oxidizing agent is electrolytically regenerated to a valence state higher than that to which it has been reduced to restore its oxidizing capacity. The regenerated ionic oxidizing agent is then recirculated in a mass transfer relationship with the gas stream. The nature of the ionic oxidizing agent and of the chemical reactions which occur in the foregoing process are described in detail in the cited patent.
The advantages of incorporating electrolytic regeneration apparatus in a system for practicing the preceding process are explained in the patent. First, regeneration is explained as being rapid and efficient. Second, some destruction of the objectionable oxidizable material occurs at the electrodes of the regeneration apparatus. Moreover, electrolytic regeneration is relatively inexpensive, it does not require additional external chemicals, operation of the apparatus can be accomplished at ambient temperatures, and this method provides a simplified means for a continuous pollution control cycle.
The electrolytic regeneration apparatus used in the process described above thus includes an electrolytic cell having one or more cathodes and corresponding, cooperating anodes. The valence of the ionic oxidizing agent decreases at the cathode of the cell as the ions accept electrons, and the valence of the ions at the anode increases as the ions release electrons. Since the purpose of the cell is to increase the valence of the ionic oxidizing agent, the rate at which the valence of the ions at the cathode is decreased must be inhibited, and this can advantageously be accomplished by making the anode area much larger than the cathode area. Prior electrolytic cells previously used for practicing the foregoing process were similar to electrolytic cells used in the art of electroplating. Accordingly, these cells comprised a set of perforated flat anode plates suspended in a tank of acid, and a set of smaller cathodes suspended in the spaces between the anode plates. The regeneration apparatus for the foregoing process should ideally regenerate the oxidizing agent at a rapid rate so as not to impede a high flow rate of liquid through the system, and with a minimal usage of power. To attain the first of these objectives, the apparatus should electrolyze the acid solution while the solution is flowing. Electrolytic cells are known for purifying flowing water. For example, U.S. Pat. No. 3,378,479 described an electrolytic cell having a mesh-like tubular anode positioned adjacent a transparent casing and a cylindrical cathode extending along the axis of the anode. U.S. Pat. No. 3,718,540 discloses an electrolysis cell which comprises tubular cathode and a generally cylindrical anode extending along the axis of the cathode and terminating in a point. Water swirls through the cell due to the arrangement of the inlet passages. Other patents disclosing similar electrolytic cells include U.S. Pat. Nos. 1,020,001, 3,457,152, and 3,528,905.
The second objective of the ideal regeneration apparatus relates to its electrical efficiency, which can be defined as the amount of oxidant produced (i.e. regenerated) per ampere of current (alternatively, such efficiency can be expressed as the coulombic efficiency, or the ratio of the amount of oxidant actually produced and the amount of oxidant theoretically produced as determined from the Faraday laws of electrochemistry). An increase in the electrical efficiency of the apparatus results in an increase in the effectiveness of the solution in treating air because of the increased amount of oxidant in the solution contacting the air. It has been found that the electrical efficiency of an electrolytic regeneration cell depends upon the uniformity of current density in the fluid being electrolyzed. The presence of edges and other irregularites in the electrodes of prior electrolytic cells has prevented uniform current density between the electrodes and has, therefore, limited their electrical efficiency.